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re: not home(turn off a/c), home( then turn on a/c) question?
25 jul 2004
timm simpkins wrote:
>> >> >...a bed that gets to 90 degrees throughout will take sometimes a week
>> >> >of a desired temperature to loose enough of its heat to be comfortable
>> >> >to sleep on again.
>> >> if your bed temp falls from 90 to 75 f ("comfortable") over a week
>> >> (168 h), in 70 f air... rc =... = 121 hours. an upper airfilm
>> >> conductance of 1.5 btu/h-f-ft^2 would make c = rc/g = 121/1.5 = 80.6
>> >> btu/h-f, eg a solid wood (hem-fir) bed 95" deep... starting from 70 f,
>> >> after 8 hours in 90 f air, the bed would reach 90-(70-90)e^(-8/121)
>> >> = 71 f.
>> >after 8 hours you should have been asleep for 4 hours on that bed that
>> >won't release its heat. once you wake up and leave your ac off for
>> >another 8 hours it's back up to the same temp again.
i'm afraid this still makes little sense to me.
>>...most beds are far less massy than the one you seem to suggest. they are
>>mostly air (the box springs), a little steel, and fabric (the mattress),
>>so they would tend to cool quickly towards 70 f in early evening.
>i wasn't using my figures. it was stated that a bed that has risen to a
>temperature of 90 degrees will reach 71 degrees after 8 hours in 70 degree
>air.
keep up :-) that was 20-minute drywall, vs your mythical 121-hour bed.
>as far as them being only air steel and fabric, yes, the box springs are
>like that, but most matresses are filled with fairly tightly packed padding,
>usually cotton i believe. that cotton has a much higher thermal mass than
>wood, steel, and fabric of the box springs.
my 60"x80"x8" queen size mattress weighs about 60 pounds. fully-packed with
cotton fiber at 95 lb/ft^3, it would weigh 95x60x80x8/12^3 = 1728 pounds.
i've never ripped it open to see what's inside, but i feel steel springs with
about 1/2" of padding on top and bottom. the ashrae hof says cotton fiber has
a specific heat of 0.319 btu/lb. the mattress might have 60x0.319 = 19 btu/f
of heat capacitance. if the box springs add another 19, we have c = 38, with
g = 1.5x2x60x80/12^2 = 100 btu/h-f (losing heat from 2 sides), rc = 0.38 h,
so this bed would heat or cool to the room air temp in about 2 hours (5 time
constants), vs a week. you seem to have trouble admitting when you are wrong,
esp to mr. speed.
>> >> >walls of a home that has been kept at higher temperatures will feel
>> >> >warm for a couple of days depending on the materials...
>> >> >walls that is constructed of water might do that. a clever thermostat
>> >> >might precompensate, keeping the room comfy with cooler air during
>> >> >that time...
>> >try walls constructed of wood. walls insulated with recycled news print,
>> >or anything else with a higher thermal mass than fiberglass batting.
>>
>> we discussed drywall. nobody mentioned fiberglass. wood doesn't have much
>> thermal capacity by volume, as you can see from the bed example. neither
>> do most insulations.
>
>actually wood is a fairly good thermal mass. i don't have the figures right
>here, but i believe it was rated at 2.78. whatever it was, it was close to
>that.
the ashrae hof says hem fir with a 12% moisture content weighs 24.5-31.4
lb/ft^3, with 0.39 btu/lb-f, so it stores about 11 btu/f-ft^3. you might
say that's "close to 2.78" :-)
>> >> >concrete floors are also excellent heat storage devices...
>>
>> ceilings are better for heat storage because they can be a lot warmer than
>> the room, especially with a low-e surface. floors can't...
>>
>> >> "direct loss houses" have mass inside outsulation. houses with low-mass
>> >> isolated sunspaces and hot massy low-e ceilings can perform better.
>> >>
>> >> >thermal mass is the great storage battery for the heat that enters in
>> >> >through the windows.
>> >>
>> >> and windows are huge holes in the heat storage bucket.
>>
>> >> >...a passive solar house can loose as little as 10 degrees overnight
>> >> >with an outside temperature in the teens.
>> >>
>> >>how much would it loose by dawn if the concrete block walls were 75 f at
>> >>dusk, with r20 insulation and 8% of the floorspace as r4 windows, on an
>> >>11 f night?
>>
>> well? iirc, you claimed to have an ee degree...
>
>i didn't ask that question...
you made a less-precise claim: "a passive solar house can loose as little
as 10 degrees overnight with an outside temperature in the teens."
>...and that has nothing to do with electrical engineering.
of course it does :-) remember all those rs and cs? hollow concrete blocks
(weighing 32 pounds at 0.16 btu/f-lb, with 128 in^2 of face area) store
about 6 btu/f-ft^2 (think 6 farads), so a 32'x32'x8' tall block house with
1024 ft^2 of walls has c = 6k btu/f... 82 ft^2 of windows have 82/4 = 20.5
btu/h-f of thermal conductance, plus 1024-82 /r20 = 47 btu/h-f of wall
conductance, making rc = c/g = 6k/67.5 = 89 hours, like this, in fixed font:
1/67.5
11 ---www------- t if t(0) = 75, what's t(16)?
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>> >> >it would take 5-6 days to release all that energy at that rate if all
>> >> >the windows were blocked off.
>>
>> can you be more quantitative?
possibly not :-)
>> >you avoid losing heat through the windows with insulated shutters. you
>> >close the shutters at night or on cloudy days...
>>
>> historically-speaking, most people quickly tire of doing that. automatic
>> systems seem preferable.
>
>where the house i'm building will be off the grid, energy efficiency is
>something that is quite the necessity. if i get tired of it, it wouldn't
>take anything to make light actuated motors to open and shut the doors.
people have been trying to do that for 30 years. oddly enough, they
haven't succeeded. the doors fail to seal well, the system is way too
expensive, and so on. good luck.
>> >okay, the passive solar house i am getting ready to build has floors made
>> >of cinder block laid on its side in parallel rows...
>> >on top of that is a thin concrete slab with holes on the north and south
>> >to allow heat to escape.
>>
>> a warm air path... how could this possibly work passively? you might make
>> the slab thicker to store more heat, altho you can't make it much warmer
>> than the room temp without cooking the occupants. massy ceilings are better.
>
>may i suggest a book? the passive solar house by james kachadorian.
that's a start.
>if you have a massy ceiling, the heat stored in the ceiling is exposed to
>the cold outside air when the sun goes down.
we need insulation over the mass.
>while it may work to a degree, it isn't very efficient.
it can be a lot more efficient than what you propose.
>...with a solar slab design, you expose the slab to the solar energy
>during the day and you insulate it from the cold during the night.
how? where's the insulation? carpeting over foamboard might help...
but then you could only heat the slab indirectly, with warm air.
>> >windows enough on the south side of the house to allow sun to shine
>> >directly on the floor in winter...
big holes in the bucket.
>> "direct loss," with miserable performance, compared to indirect gain.
>
>i don't understand where you get the direct loss from.
it's a more correct way to say "direct gain" :-)
>> >an air circulation system to circulate air through the house, including
>> >the floor and the furnace.
>>
>> is this the passive part? :-)
>
>well, i don't care how good you build a house, there is no way you can have
>everything perfectly the temperature you want it. i guess i should have
>explained that the air circulation would be a method for the air to
>efficiently circulate through the house. the only time it would be active
>would be those times the furnace needs to be turned on. otherwise, the
>circulation happens due to heat difference.
can you explain exactly how warm air naturally circulates under the floor
due to heat difference? :-)
>> >the walls will be log walls and should have an r rating of about 16-20.
>>
>> sounds like they need to be 16-20" thick, on average, vs 12" r48 sips.
>
>that depends on the wood you use. you can get anything from .8 r per inch
>for oak to 1.48 r per inch for cedars. i'm building mine out of pine which
>has a 1.35 r per inch rating. that doesn't even include the thermal mass
>gains in an area with mild climate.
ah yes. dynamic r-values :-)
>> >i could go into many other details, but it would take an entire book.
>>
>> kachadorian's. i wonder where this house will be.
still wondering.
nick
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